Compounds having absorption of a large intensity against specific light are used as color tone correcting agents for recording layers of optical recording media such as CD-Rs, DVD-Rs, DVD+Rs and blue laser recording discs, image display apparatuses such as liquid crystal display apparatuses (LCDs), plasma display panels (PDPs), electroluminescence displays (ELDs), cathode ray tube display apparatuses (CRT), fluorescence display tubes and field emission displays, CCD image sensors, CMOS image sensors; LED illuminations, and the like.
For a color tone correcting agent, light absorption property for selectively absorbing light at a required wavelength area, solubility or compatibility in organic solvents and synthetic resins that are necessary for processing into a filter or film, and stability against light and heat for maintaining performances are required.
As such color tone correcting agent, uses of a cyanine compound and a squarylium compound each having an indole backbone have been reported. For example, Patent Literature 1 discloses a squarylium compound having an indole backbone having a ferrocene group, which has fine solubility and light resistance. Furthermore, Patent Literature 2 discloses a trimethinecyanine compound having CF3 at the 5-position or NO2 at the 4- or 6-position, which shows precipitous absorption and has fine wet heat resistance and light resistance. Furthermore, Patent Literature 3 discloses a trimethinecyanine compound having CF3 and NO2 at the 5-position. However, in order to use the compounds as a color tone correcting agent for an optical filter, further improvement of heat resistance is demanded so as to withstand the processing temperature of the optical filter.